I am preparing a skill challenge for my group. On a fail branch they are going to hit a wall that they can squeeze through (group check) or can back track Athletics. What skill would I use for squeezing quickly? Would it be athletics or acrobatics? Or none at all.
I'd suggest that acrobatics is more appropriate because it has to do with flexibility and tumbling and things that would be more appropriate to squeezing.
But that's only if you want to require a skill check there. Squeezing as normal does not require a check but if a character must do so quickly or with a certain amount of finesse a check is definitely appropriate.
If you group is generally weak on acrobatics, but is strong on athletics, offering a higher DC athletics check instead might be appropriate. I could even see a case made for endurance if your characters don't mind taking some physical torture in place of finesse.
Sounds like the activity of Sqeezing through a small space fits under a type of thing called Contortion.
(From the page):
Contortion (sometimes contortionism) is an unusual form of physical display which involves the dramatic bending and flexing of the human body. Contortion is often part of acrobatics and circus acts. In general, "contortionists" have unusual natural flexibility, which is then enhanced through acrobatic training, or they put themselves through intense, vigorous and painful training to gain this flexibility.
So, as you can see, Acrobatics would be the correct skill.